This invention generally relates to electronic systems and in particular it relates to an amplifier architecture using actively phase-matched feed-forward linearization.
The performance of broadband and wireless communication systems is becoming increasingly limited by the analog signal chain because the analog portion is required to operate at higher frequencies with larger dynamic range, decreased distortion, and smaller power supply voltages. Communications receivers in particular have pushed the performance of analog components. Such base station designs currently demand the highest performance analog-to-digital converters available. Driving the received signal to these converters requires an amplifier that is characterized by dynamic range and distortion performance levels greater than the converter itself. Many receiver architectures also employ subsampling techniques that require the amplifier to maintain this linearity at frequencies approaching (and soon to surpass) 100 MHz. Attaining xe2x88x92106 dBc of distortion performance at 100 MHz (supporting a full 16-bits of resolution into the digital signal processor) is the current goal in high-performance analog design.
Future trends in communications systems will make the linearity of amplifiers even more paramount. The much discussed 3G standards for wireless communications employ a modulation methodology known as Orthogonal Frequency Division Multiplexing (OFDM). This multi-channel modulation scheme provides the key benefit of immunity to inter-symbol interference that plagues many current communications systems. The cost of this immunity is increased linearity requirements. In such systems the intermodulation distortion of an amplifier plays a particularly important role in dictating performance. Filtering in most systems serves to reduce the contribution of harmonic distortion products to the degradation of system performance. However, the intermodulation distortion products (primarily the third-order intermodulation distortion) can fall directly within band in such multi-channel systems, often corrupting adjacent channels in the spectrum. Thus the trend towards increased linearity requirements and high frequency operation combine to form a powerful motivation for developing amplifier architectures that maintain ultra-low distortion across wide frequency bands.
Many previous works have explored xe2x80x9cfeed-forwardxe2x80x9d as a means of removing an amplifier""s distortion products from its output. These techniques involved complex and precisely tuned delay paths and attenuation/gain paths in an attempt to remove all distortion products from the amplifier""s output signal.
The technique described in the prior art shown in FIG. 1 is the first to specifically target third-order distortion products. The technique drastically simplifies the analog signal processing required for effective distortion cancellation. The removal of third-order distortion is highly desirable. The magnitude of harmonic components typically decreases with increasing order. Even-order harmonic products (of which the 2nd is most significant) are readily suppressed through the use of differential signal paths. This has been done in several recent operational amplifier designs. Third-order harmonic distortion and its related intermodulation distortion are typically the most significant terms remaining. However, no recent amplifier designs, except the prior art of FIG. 1, have made anything more than incremental improvements in third-order distortion performance. This is especially true of the third-order intermodulation distortion which appears at a frequency approximately that of the input signal. While higher odd-order harmonic products also produce related intermodulation products near the input signal frequency, they are typically less significant than the third-order products. For these reasons, the technique cited in the prior art shown in FIG. 1 is an important development in analog signal processing.
The prior art technique of FIG. 1 is a feed-forward linearization technique published in the 2001 IEEE ISSCC Proceedings, Ding, Y., xe2x80x9cA +18 dBm IIP3 LNA in 0.35 xcexcm CMOS,xe2x80x9d IEEE ISSCC Digest of Technical Papers, pp. 162-163, 2001. The prior art amplifier shown in FIG. 1 includes main amplifier 20 in the main signal path; scaling amplifier 22, replica amplifier 24, and correcting amplifier 26 in the feed-forward signal path; summing node 28 which combines the main signal and the feed-forward signal; input node x; and output node Yout. The equations that describe the operation of the prior art of FIG. 1 are shown below:
out20=Ax+Axcex11x3 
out22=xcex2x 
out24=A(xcex2x)+Axcex11(xcex2x)3=Axcex2x+Axcex11xcex23x3                    out26        =                                            A                              β                2                                      ⁢            x                    +                      A            ⁢                          xe2x80x83                        ⁢                          α              1                        ⁢                          x              3                                                                        y          out                =                              Ax            +                          A              ⁢                              xe2x80x83                            ⁢                              α                1                            ⁢                              x                3                                      -                          (                                                                    A                                          β                      2                                                        ⁢                  x                                +                                  A                  ⁢                                      xe2x80x83                                    ⁢                                      α                    1                                    ⁢                                      x                    3                                                              )                                =                                    A              ⁡                              (                                  1                  -                                      1                                          β                      2                                                                      )                                      ⁢            x                              
where A is the gain of amplifiers 20 and 24, x is the input signal, xcex11 is the coefficient for the third order term, xcex2 is the gain for scaling amplifier 22, 1/xcex23 is the gain for correcting amplifier 26, out20 is the output of amplifier 20, out22 is the output of amplifier 22, out24 is the output of amplifier 24, and out26 is the output of amplifier 26.
The underlying principle behind the linearization technique introduced in the prior art of FIG. 1 is that an amplifier""s transfer function can be expressed as a Taylor series expansion of the form y=a1x2+a3x3+ . . . +anxn. Limiting the number of terms in the expansion reduces the model""s accuracy, but is often acceptable under certain circumstances. The amplifier transfer function in the above analysis includes only the fundamental and third-order terms as shown by out20 in the above equations. This is a reasonable approximation because a differential signal path suppresses the even-order harmonics (the terms with even powers of x). Furthermore, many systems include filters that attenuate the higher-order harmonics. The third-order term remains because it generates distortion products near the fundamental through intermodulation.
Another important point to note is that the third-order term varies as the cube of the input. Thus an amplifier with twice the input signal will produce eight times the third-order harmonic distortion. The system illustrated in FIG. 1 exploits this relationship.
The system includes two signal paths, the main path through main amplifier 20, and the feed-forward path through scaling amplifier 22, replica amplifier 24, and correction amplifier 26. The feed-forward path begins by scaling the input by a factor of xcex2. By using a replica amplifier 24 that is identical to the main amplifier 20, the scaled input signal produces a third-order harmonic that is xcex23 times bigger than that produced at the output of the main amplifier 20. The correction amplifier 26 reduces the level of this distortion by a factor of 1/xcex2. This equalizes the third harmonic distortion with the third harmonic component of the main amplifier output, while attenuating the fundamental component in the feed-forward signal path. When these two signal paths are combined at the summing node 28, the third harmonic component is eliminated while the fundamental is only slightly attenuated as described by the equation of yout. This is an important result because the same mechanism that produces the third-order harmonic distortion (which can often be filtered) also produces the third-order intermodulation distortion (which cannot be filtered). Note that the addition of feed-forward paths tailored to address higher-order distortion products is possible.
The prior art technique of FIG. 1 has several problems, which include:
1. The unbalanced nature of the main and feed-forward signal paths results in phase mismatch at the summing node 28. Phase mismatch causes poor distortion cancellation with increasing frequency because the signals at the main and correction amplifier outputs do not coincide in the time domain. As the phase mismatch swings from 0 degrees to 180 degrees, the efficacy of the feed-forward technique ranges from complete cancellation to complete constructive interference (where the third-order distortion actually becomes twice as large).
2. The cited prior art makes use of simple differential-pair amplifiers. This open-loop amplifier configuration relies on inherent distortion performance and does not take advantage of the linearizing properties of feedback.
3. The implementation is also tuned for operation over a narrow range of frequencies. This implies a narrow band of effective distortion cancellation.
An amplifier architecture using actively phase-matched feed-forward linearization includes: a first signal path having a scaling amplifier in series with a main amplifier; a second signal path having a replica amplifier in series with a correction amplifier; and a combining node that combines the first signal path and the second signal path. This topology places the scaling amplifier in the main signal path. By inverting the scaling factor from xcex2 to 1/xcex2, this topology retains the distortion cancellation property while balancing the two signal paths. In this case the scaling amplifier attenuates the input signal rather than amplifying it. The end result remains the same. The main amplifier output signal is lower than the replica amplifier""s by a factor of xcex2. This results in a third-order distortion component that is xcex23 times bigger at the replica amplifier output. By correcting this level down to that at the main amplifier, the fundamental component in the feed-forward path is significantly attenuated. When the two signal paths combine, the third-order distortion components cancel while the fundamental is attenuated to a far lesser degree.